THE PROBLEM THAT I AM FACING IS THAT WHEN I OBSERVE THE SIGNAL THE SOME SIGNALS ARE OKAY BUT SUM SIGNAL GETS CLIPPED OF EVEN IF THEY ARE 20% VALUE OF THE SUPPLY VOLTAGE. I HAVE CHECKED EVERY PARAMETER LIKE SLEW RATE , BANDWIDTH , GAIN.
I HAVE APPLIED THE GAIN OF 200. CAN ANYONE HELP ME WHY THIS SORT OF OUTPUT IS COMING? WHY IS THE PROVIDING UNSTABLE OUTPUT?
Hi Jignesh,
Thanks for your post. Can you please share with me the supply voltages, ref voltage, input signal, and gain? These parameters will help us diagnose the problem. Based on your description, I am assuming this is a common mode issue. You may use our INA common mode calculator to determine what voltage ranges result in linear operation. The calculator can be downloaded here: http://www.ti.com/tool/INA-CMV-CALC
For example, for an INA118 with +/-15V supplies, vref = 0V, and gain = 1, your vout min is -14.8V and your vout max is +14.2V
You may also build the circuit in TINA and share it with us: http://www.ti.com/tool/TINA-TI
-Tamara Alani
Hi Mehul,
Thank you for your reply. Based on the VCM calculator, it looks like you're operating in linear conditions. Can you share with me a schematic? You can download the model from ti.com and build this in TINA SPICE.
The INA118 model (INA118 TINA-TI Reference Design (Rev. B)) can be found here: http://www.ti.com/product/INA118/toolssoftware
The TINA Spice simulator can be downloaded here: http://www.ti.com/tool/TINA-TI
Our models allow you to simulate your circuit and identify any irregularities.
-Tamara Alani
Tahnks for Answering.
When i operate with 50 gain then also it gives same problem sinusoidal output pulse. Please find attached Circuit Diagram.
Output Series cap used for offset removing.Suggest me a solution of Linear output with 200 gain with null offset(min offset).
Send a Circuit Diagram with Circuit Connection for Solution.
Hi Mehul,
Thank you for sharing the circuit with me. If I'm reading your input signal correctly, it's a square wave with a 100µs pulse width and a period of 250ms. 100µs pulse width would indeed correlate to a 10k samples/second measurement rate but that's only assuming that the it's perfectly coherent. Try zooming in on your scope, increase the sampling speed to >10k (100k would probably to the trick).
Another suspicion would be the cap load at the output of the INA118. You have a 1µF capacitor hanging off the output. The datasheet parameters specify capacitance up to 100pF for stability.
Try these and let me know.
-Tamara Alani
Hi Mehul,
The reason your square wave input results in a sinusoidal output is because the INA is bandwidth limited. At gains of 50 or 200, when you compound this low bandwidth INA with the OPT101, the -3dB roll-off frequency gets pulled back significantly causing this distortion.
I have 2 recommendations for you.
-Tamara Alani
